Transmission lines that are used to transmit electrical energy are often characterized as, high voltage lines, extra high voltage (EHV) lines, and ultra high voltage (UHV) lines. The voltage levels in each category are approximately, under 300 kilovolts (kV), 300 kV to 500 kV, and over 500 kV, respectively. Transmission lines that are in the category of EHV and UHV are referred to as bulk power transmission lines, meaning that power is transferred along the lines from a generating point to a distribution point over great distances which can be as much as several hundred miles. Another feature of such power delivery is that total power is in the range of hundreds of megawatts delivered to the distribution point or load point. It is not uncommon to see several transmission lines, consisting of three phase conductors each, passing over a common right-of-way from the generation point to the load point. These lines may be located in open farmlands, or hilly terrain, or up and down a mountainside. In any case the uninterrupted delivery of power to the load point is a primary concern. Economy of delivery is also a major concern in the design of the line.
One way of achieving economical design at the EHV and UHV voltages is to cause the conductors to be bundled. A bundled conductor differs from a single conductor because two or more conductors are tied together by devices known as spacers, or spacer-dampers. These devices are designed to keep the individual wires in a bundle separated by a fixed distance, usually 18 inches. The spacing of the spacer devices from each other along the line is about 200 feet. Hence, a line having a span of 1000 feet will have four spacers, or spacer-dampers along its span length. Since there are three phases in a circuit, a single span of 1000 feet will have twelve spacers or spacer-dampers. Therefore, there are approximately fifty spacer units per mile, or five thousand spacer units per 100 miles of line.
Modern spacers are called spacer-dampers because they combine the function of spacing the bundle with the damping of the vibration of the individual wires in the bundle. The individual wires are called subconductors. A typical 500 kV line will have three subconductors per phase in the shape an inverted equilateral triangle (inverted delta) with 18 inches separating each subconductor. Other lines may have only two subconductors and these are also separated by spacer-dampers at 18 inches, usually in a horizontal plane. Other higher voltage lines may have four subconductors in a square box arrangement separated at 18 inch intervals. A typical voltage in the case of two subconductors is 345 kV, while a typical voltage in the case of four subconductors is 765 kV.
There are as many different ways to design spacer-damper devices as there are manufacturers that make them. Competitive cost is always a major concern. Long life in service over a period of twenty years or more is a major concern as well. Most especially, it is desired to provide a spacer-damper that performs effectively to dampen vibration of the subconductors without damaging the individual wires. Another factor which is very important is to provide a device that is easy and quick to install on the line. This usually means that a quick-acting, positive-locking bolt or clip is needed. This is especially needed when the spacer-damper units are to be contracted on the basis of lowest cost.